1. Field of Invention
The present invention relates to the field of cold neutral atom production by magnetic trapping.
2. Description of Related Art
The general principle of magnetic trapping of atoms is known. The article xe2x80x9cPermanent Magnet Trap for Cold Atomsxe2x80x9d which appeared in Phys. Rev. A 51, R22 (1995) describes for example a device having permanent magnets able to produce a very high field. The trap is charged from a slowed jet. A six-beam molasses in the heart of the trap cools down the trapped gas to a temperature of 200 microkelvins. To reach the high densities required for a high rate of elastic collisions, a very steep magnetic potential is required. The curves generated by the prior art devices are approximately 100 Gauss per cm2.
However, such devices do not enable the condensate to be extracted outside the magnetic potential by cutting off the field, nor do they allow field curvatures to be modified. It is also possible to add coils to one or more poles to compensate for remanent fields.
Such a device is described in the article xe2x80x9cTrapping Cold Neutral Atoms with an Iron-Core Electromagnetxe2x80x9d in Eur. Phys. J. D 1, 255-258.
This document discloses the use of a ferromagnetic-core electromagnet to generate the trapping magnetic field. Such devices are not entirely satisfactory because Foucault currents limit the rise time and magnetic field cutoff time. To remedy these drawbacks, devices including electromagnets with improved confining power are desired.
The invention relates to a device with additional poles comprised of an external structure and an internal structure, said poles being excited separately by two coils traversed by opposing currents. In particular, the invention relates to an electromagnetic device for producing cold neutral atoms having a ferromagnetic structure with four poles disposed in the same plane XOY excited by main coils (the quadrupole) supplying the main excitation, and two additional poles (the dipole) oriented along an axis Z and perpendicular to the plane of said four poles, the poles being magnetically coupled by one or more yokes. xe2x80x9cYokexe2x80x9d is understood to be a part made of ferromagnetic material that circulates the flux.
This device is a compensated-bias Ioffe-Pritchard trap for trapping cold atoms (Bose-Einstein condensates) and/or creating a coherent cold-atom source, to achieve high compression ratios with low power consumption. It allows continuous or pulsed operation with turn-off times of 100 microseconds. It also enables the magnetic fields produced by the various coils to be adjusted by adjusting the current flowing through the excitation coils.
Advantageously, the additional poles are formed by an essentially cylindrical core, one end of which is provided with a coaxial annular cavity whose interior accommodates the interior excitation coil.
According to a preferred embodiment, at least some of the poles have a sleeve provided with a tubular channel for circulation of a heat-regulating fluid. Said sleeve is preferably surrounded by the compensating coil.
According to one variant, the yokes are formed by two annular elements with radius Rint and Rext with Rext=Rint+E where E is the thickness of the yoke, the two elements being nestable and positioned in two perpendicular planes.
Advantageously, the yokes are formed by a first annular element extending over 180xc2x0, extended at each end by second annular elements extending over 90xc2x0, in a plane perpendicular to the plane of the first element, each of said second annular elements being coupled with an annular element extending over 180xc2x0 in a plane perpendicular to the other annular elements.
According to one preferred embodiment, the yokes and poles are made of a ferromagnetic material limiting Foucault currents, particularly a laminated ferromagnetic material or sintered materials. According to one example, the yoke has annular lamination perpendicular to the lamination of the poles. Preferably, the ends of at least some of the poles are beveled.